1. Field of the Invention
The present invention relates to prosthetic devices, including hip joint stems and knee joint fixation posts, and more particularly to prosthetic devices that are coated or covered by a bioabsorbable material to provide selective stress shielding of or attachment by the adjacent bone while the implant is healing into place.
2. Description of the Related Art
Surgeons believe that distal fixation of a pres-fit (cementless) femoral hip implant can occur too early and thereby create stress shielding in the proximal region of the femur. Fixation posts on the tibial and femoral knee implant can create similar effects. This stress shielding can lead to adverse bone resorption with its attendant adverse effects on the patient. For example, stress shielding causes the bone to retreat (resorb) from its tight fit around a prosthetic implant thereby introducing some "play" into the fit. Depending upon the type of implant, metallic, polymer composite or ceramic, this loosening can lead to wear or microfretting of the implant's surface thereby releasing wear debris into the body tissue. Not only might this wear debris have adverse health consequences for the patient, but this wear debris can lead to accelerated fretting or abrasive wear of the implant thereby aggravating the further loosening of the implant. If unchecked, this situation may ultimately result in a need for major surgery to replace the implant. Such replacement may then necessitate further bone removal to provide a suitable seat for a new implant. The cycle of implantation, bone resorption, implant loosening, and replacement may then recommence. This cycle is clearly harmful to the patient and the bone resorption effect is especially harmful if the patient is young and active since the patient may then need several replacements during his or her lifetime.
Various patents and published patent applications disclose coatings of bioresorbable polymers onto prosthetic bone implants. In some cases, these coatings were on porous surfaces of the prosthesis and were comprised of bioresorbable polymers and tricalcium phosphate which encourages bone ingrowth into the porous surface. In other cases, the bioabsorbable polymer coating is used with an implant for mending fractured bones, such as for example a bone plate. For instance, U.S. Pat. No. 4,338,926 describes a metal implant wherein a biologically absorbable synthetic polymeric coating is held under compression between a bone plate and adjacent fractured bone. The bone plate carries the stress load thereby shielding the bone from stress while the bone fracture is healing. While the fracture heals, gradually reducing the need for stress shielding, the polymeric material is gradually being absorbed into the body. As the absorption of the polymeric material approaches completion, substantially all of the stresses are carried by the healed bone. Thus, bone resorption is avoided. Another embodiment of U.S. Pat. No. 4,338,926 describes an intramedullary rod driven into the medullary canal of a fractured bone. The intramedullary rod is coated with a biologically absorbable material, the coating being in contact with the inner surface of the fractured bone. Again, the rod stress shields the bone fracture while the absorbable coating gradually leaches away transferring load back to the healing bone, but does not relate to controlling bone support of the device.
Significantly, U.S. Pat. No. 4,338,926 is not directed to solving problems of stress shielding and the attendant bone resorption associated with load bearing prostheses intended as a permanent replacement for the skeletal bone, such as hip joint stems, knee prostheses, and the like. The patent only addresses the problem of stress shielding when temporarily supportive implants, such as bone plates and intramedullary pins, are used. These could shield bone from stress to the point that the bone resorbs, becomes thinner, and is subject to greater risk of fracture. These methods do not relate to controlling bone attachment and subsequent support of the implant by the bone.
U.S. Pat. No. 4,990,161 to Kampner addresses the problem of abnormal transference of stress to the more distal areas of the bone shaft in most hip joint implants. Kampner attributes this problem to the penetration of the implant's stem down to the medullary cavity of the bone. Kampner's solution is a stemless implant. Thus, the Kampner patent is directed to an implant with a resorbable stem, i.e. the distal, bioabsorbable region and all key fixation regions, e.g. distal stem tip, screws, and tibial posts are constructed entirely of resorbable material. However, gradual elimination of these fixation areas will lead to micromotion and instability. Thus, the Kampner patent would not provide long term stability. Moreover, the Kampner patent is directed toward the use of a bioabsorbable polymer post or stem. Such polymer stems do not provide as much initial stability as a metal stem. Further, implants having entirely bioabsorbable stems and key fixation areas present the body with a large amount of degradation products the body must metabolize and tolerate.
One patent U.S. Pat. No. 4,888,023 describes an attachable distal sleeve for a total hip made of a polished metal and described as an integral part of the stem. Although the intent is to minimize bone attachment along the side of the sleeve because of the polish, the sleeve is permanent, and will not necessarily eliminate bone support below the distal tip.
In the case of permanent skeletal replacement load bearing implants, a section of the bone-implant interface may carry most of the imposed load while the rest of the adjacent bone is shielded from the load. The stress-shielded bone is subject to the resorption phenomenon, as explained above for the case of hip joints. Often by the very nature and shape of an implant, a certain amount of stress shielding of some adjacent bone is almost inevitable. Therefore, a technique of controlling stress or load shifting over a period of time on selected areas of the affected bone is needed to reduce or eliminate stress-shielding induced bone resorption. Preferably, this technique should not involve subjecting the patient to surgery to correct the stress shielding effect.